A multi-layer printed circuit board and a method of making the same having reduced interfacial shear stresses.
Multi-layer printed circuit boards, as the term has come to be known in the art, are used extensively today in an effort to reduce the size and number of chips, as well as external leads, required by complex microelectronic systems. The most commonly used multi-layer printed circuit board is a xe2x80x9cdouble-sidedxe2x80x9d board having top and bottom metallic layers separated by a middle layer of metal or fiberglass-reinforced epoxy. Another type of circuit board may be considered in which both the top and bottom layers have conductor patterns formed on a moddle metallic layer which are connected through the use of xe2x80x9cair bridgesxe2x80x9d by selectively etching portions of the middle layer. The bottom layer, in turn, is typically affixed to a base material such as, for example, an epoxy glass substrate, generally referred to as FR4, or an aluminum base plate having a dielectric layer to prevent shorting.
A process for manufacturing such multi-layer printed circuit boards is shown in substantial detail in U.S. Pat. No. 4,404,059 issued to Livshits et al. Livshits teaches that conductor patterns may be formed into double-side printed circuit board through the use of an additive procedure referred to in the art as xe2x80x9cRITM.xe2x80x9d As disclosed, conductor metal is electroplated onto opposed major surfaces of a substantially planar metal substrate through respective protective masks which correspond to the desired conductor pattern. The protective masks are typically photoresists which are applied by known two-side photolithography over the metal substrate. The conductor pattern on the first major surface includes bridging elements having enlarged ends and a constricting portion there between. The conductor pattern on the opposed major surface includes elements oriented transverse to the bridging elements.
After the outer conductors are electroplated on the middle metallic layer, the protected masks are removed and following a pre-etching step, an adhesive layer comprising an insulated material is secured to one major surface of the plated substrate such that it becomes embedded in one major surface of the adhesive layer. The other major surface of the adhesive layer is thereafter secured to a base. The entire substrate is then immersed into an etchant for a sufficient period of time to remove exposed portions of the substrate below the constricted portions of the bridging elements throughout the entire thickness of the substrate.
Livshits et al. suggests that a double-sided printed circuit board may also be formed by selectively removing conductor metal pre-applied over the entire surface of the substrate with the advantage that pre-fabricated bimetallic laminates prepared by metallurgical cladding techniques may be used. Livshits cautions, however, that the resulting conductors will suffer from the irregularity of edges and low produceability of shape. Accordingly, the additive technique discussed above is proclaimed as more efficient from the standpoint of conductor metal consumption as well as the attainment of higher density of the conducting pattern of the panel.
While the use of multi-layer printed circuit boards formed by the process described above have greatly advanced the microelectronics art, they have nonetheless proven unreliable and thus undesirable in superintegration applications wherein the conductor layers have thermal expansion rates which differ from the thermal expansion rate of the plastic base.
Today, most of the microelectronics industry uses epoxy glass (FR4) as a base material because it has a thermal expansion rate, also called Coefficient of Thermal Expansion (CTE), of about 17 ppm degree Celsius. This means that for each degree Celsius, the material moves 17xc3x9710xe2x88x926 inches per inch. This is similar to the thermal expansion rate as copper, a metal typically used for the top and bottom conductors of the substrate. In automotive instrument panels, where large temperature swings are present, however, substantial interfacial shear stresses occur on the printed circuit board resulting in shorting.
More specifically, in these applications where temperature requirements are xe2x88x9250xc2x0 C. to 105xc2x0 C., it is known that the plastic base will expand much more rapidly than the copper and even more rapidly than, for example, ceramic chips such as Ball Grid Arrays (BGA""s) which have even lower rates of thermal expansion (on the order of 2-8 ppm degrees Celsius). Such mismatches have precluded the use of multi-layer printed circuit boards of the type described above for use in such applications.
Consequently, a need exists for an improved multi-layer printed circuit board and method of making the same having reduced interfacial shear stresses so as to be acceptable for use in applications with large temperature gradients.
It is an object of the present invention to provide a method of manufacturing an improved multi-layer printed circuit board adapted for reduced interfacial shear stresses.
In carrying out the above object, there is provided a laminate substrate having a top layer forming a first major surface, a middle layer having a predetermined thickness, and a bottom layer forming a second major surface opposed to the first major surface. Each of the layers comprises material etchable in a respective given etchant. Conductor patterns are thereafter etched on both the first and second surface by disposing respective etch resists (photoresists), each corresponding to a reverse image of a desired conductor pattern. Thereafter, the first and second surfaces are etched in the given etchant to form the first and second conductor patterns. The first and second etch resists are then removed.
According to the invention, a compliant adhesive layer comprising a low modulus material is then secured to the second surface so that the second layer becomes embedded in one major surface of the adhesive layer. The other major surface of the adhesive layer is thereafter secured to one major surface of a base. By selectively etching the middle layer in its corresponding etchant, the portions of the first and second surfaces are thereafter isolated and interconnect regions are defined therebetween having a height equal to the predetermined thickness of the middle layer.
In keeping with the invention, the low modulus material of the compliant adhesive layer and the thickness of the middle layer are selected such that their combination provides the reduced interfacial shear stresses.
In a preferred embodiment, the laminate substrate is substantially planar and is formed by known metallurgical cladding techniques. The base substrate is also preferably, but not necessarily, comprised of a plastic material for use on automotive instrument panels which may be subject to large temperature gradients. Other base materials such as, for example, flexible sheet metal, dielectric materials, ceramic glass, polyethylene film, mylar film, PET and other continuous and discontinuous composite materials may be used depending upon the desired application.
In carrying out the above method, a dual-layer printed circuit board is formed which is adapted for reduced interfacial shear stresses. The printed circuit board comprises a laminate substrate having a top layer, a middle layer and a bottom layer. The top layer forms a first major surface having a first conductor pattern formed thereon. The bottom layer forms a second major surface opposed to the first major surface and having a second conductor pattern formed thereon. The middle layer comprises a plurality of interconnect regions or thermal posts which separate isolated portions of the first and second surfaces.
The laminate substrate further includes a compliant adhesive layer comprised of a low modulus material having a first major surface secured to the second major surface of the substrate, and a second S major surface. Finally, the printed circuit board includes a base, typically, but not necessarily, a plastic substrate, having a first major surface secured to the second major surface of the adhesive layer. Again, in keeping with the invention, the height of the thermal posts and the low modulus material of the adhesive layer are selected and combined so as to provide said reduced interfacial stresses.
These and other objects, features, and advantages of the present invention will become more readily apparent when viewed in connection with the accompanying drawings wherein like reference numerals correspond to like components.